I assume you're talking about pre-boil volumes here. It took me several batches to get my water and wort volumes under control and develop calibration numbers for my system so that I could hit my targets fairly accurately. Grain absorbs water. More grain = more absorption. The only conclusion I can draw is that you weren't using 8gal before.

A quick check here shows that using 8 gallons of mash/sparge water on 11 lbs of grain should yield 6.25 gallons of wort. So you're latest batch appears to be spot-on. My guess is that you either weren't measuring your water volumes correctly before, or you were leaving a bunch of wort in the mash tun. Every batch you get better

I have to agree with duboman. I've done multiple batches of the same recipe but the second time I brewed it, even through I kept the boil at the same rate and the amount of water/grains the same, I still ended up with a lot less liquid at the end merely because of the ambient temperature. Granted, it was quite an extreme difference, the first time I did this recipe it was 15 degrees outside and the next time it was 70--as I get to brew in an unheated shed, but it was something I had not taken into consideration the second time around until it was too late. Not to be off topic, but I do love winter brewing though as cooling the wort doesn't get much faster when you can stick the hot pot into a snowbank.